Device for early detection of rupture of the pressure part of a boiler

ABSTRACT

A device for early detecting any abnormal conditions of a boiler from the sound wave which is generated when water or steam spouts from the pressure part of the boiler. The device is adapted for use in an atmosphere, such as a recovery boiler, which is liable to contamination, and comprises a gate circuit for shutting a sound wave transmission tube and shutting an output of a microphone upon actuation of soot blowers of the boiler so as to protect the device against detrimental effects imposed thereon by the operation of the soot blowers, means for cleaning the transmission tube by passing pressurized air therein while the output of the microphone is being shut down, means for cooling the sound wave transmission tube, means for detecting an abnormal condition of the microphone by passing pressurized air in the vicinity of said microphone and means for detecting clogging of the sound wave transmission tube by pressure.

United States Patent Yamamoto et al.

[11] 3,831,561 [451 Aug. 27, 1974 [75] Inventors: Takashi Yamamoto, Togitsu-machi;

Kenichi Yasukouchi; Ryuichi Sato, both of Nagasaki, all of Japan [73] Assignee: Mitsubishi Jukogyo Kabushiki Kaisha, Tokyo, Japan [22] Filed: Sept. 14, 1973 [2]] Appl. No.: 397,355

[30] Foreign Application Priority Data 6/1965 Simpkins et al. 340/242 12/1965 Simpkins ct a1. 340/242 X Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-Cushman, Darby & Cushman ABSTRACT A device for early detecting any abnormal conditions of a boiler from the sound wave which is generated when water or steam spouts from the pressure part of the boiler. The device is adapted for use in an atmosphere, such as a recovery boiler, which is liable to contamination, and comprises a gate circuit for shutting a sound wave transmission tube and shutting an output of a microphone upon actuation of soot blowers of the boiler so as to protect the device against detrimental effects imposed thereon by the operation of the soot blowers, means for cleaning the transmis sion tube by passing pressurized air therein while the output of the microphone is being shut down, means for cooling the sound wave transmission tube, means for detecting an abnormal condition of the microphone by passing pressurized air in the vicinity of said microphone and means for detecting clogging of the sound wave transmission tube by pressure.

8 Claims, 10 Drawing; Figures PAIENIED B2 974 SEE! 3W 5 DEVICE FOR EARLY DETECTION OF RUPTURE OF THE PRESSURE PART OF A BOILER BACKGROUND OF THE INVENTION In a soda recovery boiler in which a pulp digesterliquid is burned to recover chemicals and heat, the pulp digester liquid is discharged into the boiler furnace from a black liquid discharge nozzle of the soda recovery boiler in the form of fine particles. The water and volatile components contained in the pulp digester liq uid evaporate and the liquid is decomposed, while the fine particles of the liquid fall to the boiler furnace bottom, forming a char bed at the furnace bottom which consists essentially of carbon and inorganic chemicals. The inorganic chemicals in the char bed melt and form a so-called smelt pool beneath the char bed.

In the event that water intrudes into the soda recovery boiler furnace for any reason, the water evaporates explosively, and further, a water gas reaction occurs in the char bed and the following reaction between the smelt and water occurs in the smelt pool:

5 4H Na- SO +4H This reaction is usually accompanied by an explosion more intense than the rapid explosive evaporation of water. Therefore, if a rupture of the boiler pressure part is the cause of penetration of water, even in a slight amount, into the boiler furnace, the opening of the rupture will be enlarged by the pressurized water or steam gushing therethrough and the amount of water or steam intruding into the boiler furnace gradually increases, inducting further small explosions and making the rupture opening larger and larger, with the consequence that a serious explosion is induced which not only causes heavy damage to the soda recovery boiler itself but also endangers the operators of the boiler. Thus, it becomes necessary to stop the operation of the soda recovery boiler immediately upon intrusion of waterinto the boiler furnace. For preventing the explosion of the soda recovery boiler caused by the intrusion of water into the boiler furnace, it is most important to make a careful inspection and necessary repair of the boiler prior to the operation and also to train the operators to operate the boiler in a way which precludes accidents.

However, if water unfortunately has intruded into the boiler furnace, the intrusion of water should be detected as early as possible and the operation of the boiler should be stopped at once to minimize the chance of accident. Nevertheless, no attempt has been made in the past to provide a method by which the intrusion of water or steam into the soda recovery boiler can be positively detected and, although a considerable number of these types of accidents have been experienced in the United States and Canada, theabnormal condition of the boiler has always been detected first by the operator by inspection of the boiler furnace interior upon sensing an abnormal sound in various noises occurring during the operation of the soda recovery boiler, e.g., the sound of continuous small explosions in a smelt dissolving tank, the combustion noise in the boiler furnace and the operating noises of induced and forced draft funs.

Abnormal conditions of the boiler caused by water or steam intruding into the boiler furnace from the pressure part of the boiler, include abnormal sound waves andvibrations occurring incident to the intrusion of the water or steam, and, when the boiler is a soda recovery boiler, an increase of the steam partial pressure in the furnace gases and the generation of hydrogen due to the reaction between the smelt deposited on the furnace wall and the spouting water or steam.

It is practically impossible to utilize the steam partial pressure increase and the generation of hydrogen gas, of the abnormal conditions mentioned above, for the early detection of the water or steam spouting from'the pressure part of the recovery boiler. This is because the increase ratio of the steam partial pressure or hydrogen partial pressure is verysmall since the amount of the furnace gases is extremely large, and because the steam partial pressure in the furnace is originally high and a fluctuation of the steam occurring incident to the fluctuations of the concentration and amount of the black liquid is large since the black liquid injected into the soda recovery boiler furnace from the black liquid discharge nozzle is undried and contains a large amount of water.

It is considered that a rupture of the boiler pressure part can be detected early by sensing the abnormal vibrations occurring incident to the spouting of water or steam from the boiler pressure part. Namely, whena rupture has occurred at the pressure part of the boiler for any reason and the water or steam spouts from the opening of said rupture at sonic or supersonic speed, abnormal sound waves and vibrations are generated at or in the vicinity of the rupture opening. The frequencies of the abnormal sound waves and vibrations are as high as 20 KH and the vibrations in such a high frequency range are propagated in steel member connected to the pressure part of the boiler and reach a detection end provided at a suitable position of the boiler structure. The detection end detects the abnormal vibrations transmitted directly in the steel member.

In a boiler, an operation called soot-blow, i.e., blowing the boiler with pressurized steam, is periodically performed to remove soot and other substances deposited on the wall of water tubes. The amount of pressurized steam used for soot-blow is so large as compared with the mount of steam to be detected spouting from the rupture at the pressure part that the detection of the abnormal vibrations is extremely difficult while being interfered by soot-blow. One boiler is usually provided with 10 to 20 soot blowers which are operated sequentially (the operating time of each soot blower being 2 to 10 minutes) to complete one cycle of sootblow in a period of l to 2 hours. In an ordinary utility boiler, two to three cyclic operations of the soot blowers once every day are sufficient for soot-blow and a detecting device for detecting the vibrations or sound wave generating incident to the spouting of water or steam can operate normally during the rest of the time. However, in case of boilers, such as a soda recovery boiler, in which the amount of depositing soot is large, soot-blow must be carried out continuously almost all day long, and in case of the soda recovery boiler in particular, the correct detection of water or steam leakage is quite difiicult, despite the fact that water or steam leakage ,will possibly result in a serious accident.

In view of the foregoing, there can be considered the use of such a detecting device which is provided with a gate circuit which will open'to transmit an output of the detecting device only when the soot blowers are not in operation, and which therefore, can even be used with such a boiler which calls for continuous soot-blow almost throughout the period of operation, for detecting periodically a leakage of water or steam due to a rupture of the pressure part to preclude detrimental effects otherwise induced by the leakage.

However, as a result of the experiment conducted by the present, inventor using a soda recovery boiler of the type shown in FIG. 1, in which a pressure pipe 01 having an 0.5 mm diameter opening intentionally formed therein is connected to an inlet of an economizer and a detection head of a vibration detecting device of the type described is attached to a position 02 less than 5 m distant from a discharge opening of a member connected to said pressure pipe, and by causing water and steam to spout from said opening during the operation of the boiler,'it was found that the frequency spectrum of the vibration is substantially the same as the frequency spectrum of the normal vibration as shown in FIG. 2, and the abnormal vibration caused by the spouting water and steam cannot be detected. This means that the normal vibration of the boiler during operation is considerably large over the range from low frequency to a high frequency of KH as compared with the abnormal vibration caused by the spouting of water or steam, depending upon the position of the detection head, and the abnormal vibration cannot be detected by filtering the normal vibration of the boiler, so that the positive detection of the spouting of water or steam from the vibration transmitting in the boiler structure is difficult.

Nevertheless, it is essentialin case of a failure of the boiler pressure part, to detect a leakage of the degree mentioned above as early as possible and to take the necessary measure for preventing explosion.

On the other hand, the sound wave which is generated incident to the spouting of pressurized water or steam through a rupture opening formed in the pressure part of the boiler, for any reason, at the sonic velocity or supersonic velocity, expands within the boiler furnace and fills the interior of said furnace while reflecting on a water-cooled pipe surrounding the furnace. Part of the sound wave is absorbed by the watercooled pipe and other members connected to the furnace, but the other part propagates in air and reaches a sound wave detector or the like provided at a suitable location of the furnace wall. This sound wave is at a level equal to or several times that of the dark ground noise of the boiler in the frequency range from a low frequency of about 3 KH to a supersonic frequency of 20 KH Therefore, the spouting sound can be detected relatively simply from the dark ground noise of the boiler by detecting the sound wave and passing it through a sound filter of a suitable frequency range. In this way, it becomes possible to detect a slight water or steam leakage which cannot be detected from the vibration transmitted in the boiler structure. To prove this, the present inventor conducted an experiment in which a pressure pipe (indicated at numeral 01 in FIG. 1) having an 0.5 mm diameter opening intentionally formed therein was connected to an inlet of an economizer, in exactly the same manner as in the experiment for detecting vibrations described above, and a detection head of a sound wave detecting device was attached to a manhole 03 of the economizer, and then water and steam were caused to spout from said opening during the operation of the boiler, with the result shown in FIG. 3. It will be seen that the frequency spectrum of the sound generated by the spouting water or steam reaches a level several times higher than that of the boiler dark noise in the frequency range from 30 to 20 KH This shows that the spouting sound of water or steam can be sufficiently detected by passing the sound through a filter of the frequency range of 3 to 20 KH In the preclusion of explosion in a soda recovery boiler, there is more necessity for detecting a water and steam leakage at the furnace part of overheater part than for detecting the leakage at the economizer part,

as stated previously and thus it becomes necessary to provide further that the leakage at the furnace part and the superheater part can be detected. However, it is impracticable to cause water or steam to spout at the furnace part of the actual boiler because such practice involves the danger of explosion. Therefore, the present inventor at first made an attempt to confirm through experiment that air can be used in lieu of water and steam, and found that the frequency spectrum of the sound wave generated by pressurized air of 5 kg/cm spouting from a 5 mm diameter opening is very close to the frequency spectra of the sound waves generated by saturated water and saturated steam of a pressure of kg/cm spouting from an 0.5 mm diameter opening, and that air is well usable as a substitute for water and steam. Based on such knowledge, the present inventor conducted an experiment using air over the entire region of the boiler, and confirmed that air can be sufficiently usable even in the actual boiler. Namely, compressed air of a pressure of 5 kg/cm wasdischarged from a 5 mm diameter opening at the position of each of a starting burner nozzle (indicated by numeral 04 in FIG. 1) and a black liquid spray nozzle (indicated by numeral 05 in FIG. 1) and the pressure levels of the generated sounds in the frequency range from 3 to 20 KH were sensed by a sound detection end 07 provided at the outlet of the furnace and recorded on linear coordinates, with the result shown in FIG. 5. The sounds had sufficient sound pressure levels as indicated at a and b in FIG. 5. This also means that spouting of water or steam from an 0.5 mm diameter opening can be well detected.

SUMMARY OF THE INVENTION The present invention has for its object the provision of a device adapted for use with a general boiler for detecting promptly a spouting of water into the boiler furnace due to rupture of the pressure part of the boiler and thereby for minimizing the chance of accident, the device when used with a soda recovery boiler enabling the operator to detect the intrusion of water or steam into the boiler furnace early and positively, to stop the boiler upon occurrence of such condition and thereby to preclude a dangerous accident of the boiler.

Another object of the invention is to provide a device of the character described above, which is provided with means for enabling the device to maintain its normal detecting function without being affected by the operation of soot blowers.

Still another object of the invention is to provide a device of the character described above, which is provided with means for cleaning the device with pressurized air so as to enable the device to maintain its normal detecting function even when used in an atmosphere liable to contamination, such as a soda recovery boiler.

Still another object of the invention is to provide a device of the character described above, which is provided with means for determining whether a microphone to detect a sound wave generating as a result of rupture of the pressure part is in the normal operative state or not.

A further object of the invention is to provide a device of the character described above, which is provided with means for detecting a clogging of the device due to contamination soda to ensure the satisfactory function of the device even when the device is used in an atmosphere liable to contamination.

An additional object of the invention is to provide a device of the character described above, which is thermally protected and has a high detection sensitivity.

Other objects and advantages of the invention will become apparent from the following description of an embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 5 are diagrams for explaining the background of the present invention, of which:

FIG. I is a diagram for explaining a setup which was used for examining the boiler pressure part rupture detecting effect of the subject device in a conventional soda recovery boiler;

FIG. 2 is a diagram exemplifying a comparison between the frequency spectra of the normal vibration of the boiler and an abnormal vibration of the same due to rupture;

FIG. 3 is a diagram exemplifying a comparison between the frequency spectra of the dark ground noise of the boiler and an abnormal noise of the same due to rupture;

FIG. 4 is a diagram exemplifying a comparison between the frequency spectra of the water, steam and air gushing sounds;

FIG. 5 is a diagram exemplifying a recorded air gushing sound at the boiler furnace part;

FIG. 6 is a view exemplifying the position of the device according to the present invention as applied to a soda recovery boiler;

FIG. 7 is a block diagram illustrating briefly the construction of the device according to the invention;

FIGS. 8 and 9 are views exemplifying a preferred embodiment of the device of the invention; and

FIG. 10 is a flow sheet for explaining the operation of the device of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The device according to the present invention will be described herein as applied to a soda recovery boiler, with reference to the drawings. FIG. 6 shows a portion of the soda recovery boiler, in which reference numeral 1 designates a boiler furnace tube, 2 a superheater, 3 a boiler tube, 4 an economizer, 5 manholes, 6 a peep hole and 7 the location of a sound wave detection head.

FIGS. 8 and 9 exemplify the practical construction of a fixed type sound wave detection head which is to be disposed, for example, at the position 7 in FIG. 6.

The detection head generally indicated by numeral 11 comprises an air-cooled sound wave transmission tube 12, an electric three-way valve 13, a sound wave transmission tube 14 and a microphone 15. Since this detection head 11 is left mounted on the furnace wall, particular consideration is given thereto to render it resistive to heat and dust proof.

A cooling chamber 16 is provided around the sound wave transmission tube 12 for cooling tube 12 with air. The provision of the cooling chamber 16 is advantageous in reducing the length of the transmission tube 12 and achieving detection with high sensitivity, without attenuating a high frequency sound wave to be detected.

Reference numeral 17 designates a cooling air inlet tube; 18 a pressure regulator for the flow rate of the cooling air so as to eliminate detrimental effect on the detecting device of the noise which would be generated by the flow of cooling air when the: flow rate of cooling air is excessively high; 19 a reference leakage sound generating hole for generating a reference leakage sound which is used for checking a performance degradation of the microphone or an abnormal operation of the detecting device; 20 an inlet tube for leakage sound generating air; 21 an electric air valve for opening and closing the leakage sound generating air passage; and 22 a branch tube diverging from the cooling air inlet tubel7.

In order to prevent the inlet opening of the sound wave transmission tube 12 from being clogged with soot attaching to the inner wall of the furnace at the portion where the detection head is mounted, a flashing air inlet chamber 23 is provided for injecting flashing air into the sound wave transmission tube 12 in four directions. Reference numerals 24 and 25 designate a flashing air inlet tubes. The electric three-way valve 13 is provided between the sound wave transmission tubes 12 and 14, which severs communication between the sound wave transmission tubes 12 and 14 to shut down the transmission of sound wave to the microphone 15 and establishes communication between said sound wave transmission tube 12 and the flashing air inlet tube 25, at the time of soot blow, providing for passage of flashing air in said flashing air inlet tube 25 and sound wave transmission tube 12 through an electric air valve 26 which is then open.

A pressure sensor 28 is provided for issuing an alarm in the event when the sound wave transmission passage has clogged with soot attaching to the inlet portion of the sound wave transmission tube 12. This pressure sensor 28 is designed such that it will issue an alarm when the pressure difference between the internal pressures of the sound wave transmission tubes 12 and the furnace, or the internal pressure of the sound wavetransmission tube 12 reaches a certain level. Reference numerals 27 and 29 designate tubes for supplying the internal pressures of the furnace and sound wave transmission tube to pressure sensor 28..

Reference numeral 30 designates a bracket for mounting the microphone on the sound wave transmission tube 14, which is made of a heat insulating material; 31 an Oring; 32 a cooling air outlet tube; 1 a furnace cooling water tube; 34 a heat insulating material and 35 a housing.

The forward end of the sound wave transmission tube 12 has the shape of an exponential'horn for damping waves of low frequencies below about 3 KH FIG. 10 shows a block diagram of an example of the detecting device of the invention for explaining the operation principle of the device. The detecting device shown comprises a band-pass filter 36, a gate 37 opened and closed in response to signals from control means 38, a monitor and a soot blower controller which generates signals for actuating and stopping soot blowers.

As stated previously, the operation of the soda recovery boiler is accompanied by heavy attachment of soot and calls for repetitive soot-blow operations. Therefore, the detection of the leakage sound has to be made in the interval between the soot-blow operations. When a certain soot blower is set in operation for a period of 2 to 10 minutes, the electric three-way valve 13 is actuated by the signal from the controller 38 and the sound wave transmission passage leading to the microphone is shut down by said valve, to prevent overloading of the microphone. Then, the electric valve 26 is opened to flash the sound wave transmission passage. In this case, the gate 37 is held closed to shut down the transmission of a signal from the band-pass filter 36 to the monitor 39 and thereby to prevent erroneous operation of the detecting device otherwise caused by the noises from the soot blower and flashing air flow.

The electric valve 26 is closed concurrently with the termination of the operation of the first soot blower and, therefore, flashing of the sound wave transmission passage ends. Then, the electric three-way valve 13 is actuated to open the sound wave transmission passage leading to the microphone l5 and the gate 37 is also opened. If water or steam is spouting through a rupture formed in the boiler pressure tube'in this case, such condition is detected by the sound wave detection head 11 and transmitted to the monitor 39 through the bandpass filter 36 in the form of a high frequency signal.

Then, the second soot blower is set in operation, whereupon the electric three-way valve 13 is actuated to shut down the sound wave transmission passage leading to the microphone and concurrently the gate 37 is closed and the electric valve 26 is opened, so that the sound wave transmission passage is flashed again. The interval between the termination of operation of the first soot blower and the start of the second soot blower is only about 30 to 100 seconds. With the detecting device of the invention, however, it is possible to monitor the boiler periodically all day long by making use ofsuch a short period of time.

The detecting device of the invention, as stated above, is provided with pressure sensor 28 for detecting clogging of the inlet portion of the sound wave transmission passage l2 by soot. This pressure sensor 28 transmits a signal to the controller informing it of heavy deposition of soot, when the internal pressure of the sound wave transmission tube 12 or the pressure difference between the internal pressures of said sound ave transmission tube and the furnace, at the time of sootblow, has reached a preset value;

When it is desired to check the performance of the microphone or the operation of the detecting device, the electric three-way valve 13 is actuated to close the sound wave transmission passage leading to the microphone and the electric valve 21 is opened, either automatically from the controller 38 or manually, at a suitable time when the soot blowers are not in operation, and thereby air is caused to discharge from the reference leakage sound generating hole shown in FIG. 8 to generate the reference leakage sound, and concurrently the gate 37 is opened. An alarm is issued indicating the performance of the microphone being degraded or the operation of the detecting device being abnormal, when the leakage sound is below a preset level.

In the event when water or steam has intruded into the furnace, the overheater or the economizer of the soda recovery boiler upon occurrence of a rupture in the pressure tube and an abnormal sound wave has been generated, the abnormal sound wave spreading in air, upon reaching the microphone 15, is immediately converted into an electric signal which is discriminated from the electric signal, created from the dark noise of the boiler, by the band-pass filter 36 and transmitted to the monitor 39 through the gate 37 which is then open.

The monitor 39, upon receiving such electric signal, transmits a signal to alarm means, such as a buzzer or a lamp, to actuate the same, to means for automatically stopping the boiler, or to the soot blower controller 40 for stopping the soot blowers only. Alternatively, an arrangement may be made such that the monitor, upon receiving the electric signal, will continue its monitoring operation for a while and transmit the alarm actuating signal or boiler stopping signal after confirming that the boiler is in a dangerous condition.

It is also possible to preset in the monitor 39 the frequency, the amplitude or the duration of an abnormal sound wave which will be received by the monitor and which indicates that the boiler is in the dangerous condition.

The detecting device of the invention, as has been described in detail by way of example herein, monitors constantly an abnormal condition of the boiler by detecting the sound wave, generating from water or steam leaking from the boiler pressure tube and spreading in air or trapped in the furnace, by the detection head attached to the furnace wall, converting the sound wave into electric signals and discriminating the electric signal from the signals originating from the dark ground noises of the boiler by the band-pass filter. With the device of the invention, it is possible to detect the occurrence of a rupture of the pressure part, such as the water tube, of the boiler early and positively, and upon detecting the abnormal sound wave, the device converts it into an electric signal, or pneumatic or hydraulic signal which is transmitted to the alarm means or explosion preventing means to actuate the same automatically, whereby the occurrence of explosion in the boiler furnace can be avoided and thereby heavy damages to the facility as well as persons can be avoided which would otherwise be caused by the explosion.

Many changes and modifications in the abovedescribed embodiment of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, that scope is intended to be limited only by the scope of the appended claims.

What is claimed is:

l. A device for early detecting a rupture of the pressure part of a recovery boiler having soot blowers, comprising:

a. a sound wave transmission tube open at its forward end in the furnace of the recovery boiler and connected at its rear end through a three-way valve to a conduit having a microphone disposed therein,

b. means for applying the output of said microphone to a band-pass filter and the output of said filter to a gate circuit which produces an output signal indicating rupture,

0. means for injecting pressurized air into the rear end of said sound wave transmission tube through a further valve and said three-way valve,

d. means for injecting pressurized air into the open end portion of said sound wave transmission tube adjacent the wall of said furnace of the recovery boiler through said further valve, and

e. control means for closing said gate circuit, opening said further valve and operating said three-way valve to communicate said sound wave transmission tube with said further valve before the start of soot-blow of said recovery boiler, and for opening said gate circuit, closing said further valve and operating said three-way valve to communicate said sound wave transmission tube with said conduit after the termination of soot-blow.

2. A device according to claim 1, in which means is additionally provided for injecting pressurized air into said conduit from a nozzle through another valve.

3. A device according to claim 1, in which means is provided for detecting a pressure difference between the pressure in the rear end portion of said sound wave transmission tube and the pressure in the other end portion of said sound wave transmission tube open in said furnace.

4. A device according to claim -1, in which means is provided for cooling said sound wave transmission 10' tube.

5. Apparatus for providing an early indication of rupture of the pressure part of a recovery boiler having a plurality of soot-blowers;

a sound tube transmission tube open at its forward end in the boiler,

microphone means for detecting an acoustical wave in said tube and producing an electrical signal,

a band-pass filter connected to said microphone means and receiving said electrical signal,

means for indicating an alarm,

electrical gate means connected between said alarm indicating means and said band-pass filter for passa ing the filtered signal in a closed condition and blocking the filtered signal in an open condition and control means connected to said soot-blowers for causing said gate means to be in said closed condition only while said soot-blowers are not operating.

6. Apparatus as in claim 5 including means for detecting a pressure differential between the pressure in the rear portion of said tube and the portion of said tube open in said boiler.

7. Apparatus as in claim 5 further including means for supplying a stream of air into said open end of said tube when said soot blowers are operating to prevent clogging.

8. Apparatus as in claim 5 further including means for providing a flow of air into said tube adjacent said microphone means to simulate rupture and check said 

1. A device for early detecting a rupture of the pressure part of a recovery boiler having soot blowers, comprising: a. a sound wave transmission tube open at its forward end in the furnace of the recovery boiler and connected at its rear end through a three-way valve to a conduit having a microphone disposed therein, b. means for applying the output of said microphone to a bandpass filter and the output of said filter to a gate circuit which produces an output signal indicating rupture, c. means for injecting pressurized air into the rear end of said sound wave transmission tube through a further valve and said three-way valve, d. means for injecting pressurized air into the open end portion of said sound wave transmission tube adjacent the wall of said furnace of the recovery boiler through said further valve, and e. control means for closing said gate circuit, opening said further valve and operating said three-way valve to communicate said sound wave transmission tube with said further valve before the start of soot-blow of said recovery boiler, and for opening said gate circuit, closing said further valve and operating said three-way valve to communicate said sound wave transmission tube with said conduit after the termination of soot-blow.
 2. A device according to claim 1, in which means is additionally provided for injecting pressurized air into said conduit from a nozzle through another valve.
 3. A device according to claim 1, in which means is provided for detecting a pressure difference between the pressure in the rear end portion of said sound wave transmission tube and the pressure in the other end portion of said sound wave transmission tube open in said furnace.
 4. A device according to claim 1, in which means is provided for cooling said sound wave transmission tube.
 5. Apparatus for providing an early indication of rupture of the pressure part of a recovery boiler having a plurality of soot-blowers; a sound tube transmission tube open at its forward end in the boiler, microphone means for detecting an acoustical wave in said tube and producing an electrical signal, a band-pass filter connected to said microphone means and receiving said electrical signal, means for indicating an alarm, electrical gate means connected between said alarm indicating means and said band-pass filter for passing the filtered signal in a closed condition and blocking the filtered signal in an open condition and control means connected to said soot-blowers for causing said gate means to be in said closed condition only while said soot-blowers are not operating.
 6. Apparatus as in claim 5 including means for detecting a pressure differential between the pressure in the rear portion of said tube and the portion of said tube open in said boiler.
 7. Apparatus as in claim 5 further including means for supplying a stream of air into said open end of said tube when said soot blowers are operating to prevent clogging.
 8. Apparatus as in claim 5 further including means for providing a flow of air into said tube adjacent said microphone means to simulate rupture and check sAid microphone means. 